Process for producing a first electrode and a second electrode, electronic component and electronic memory element

ABSTRACT

In a process for producing a first electrode an- a second electrode, the first electrode and the second electrode are provided on an electrode material. A cluster ion source is used to apply clusters of the electrode material to the first electrode and/or the second electrode.

[0001] The invention relates to a process for producing a firstelectrode and a second electrode, to an electronic component and to anelectronic memory element.

[0002] For some applications in the microelectronics sector, it isdesirable for the surface area of electrodes, for example of siliconelectrodes or also metal electrodes, to be increased, in order, in thisway, to achieve the maximum possible capacitance of the electrodes.

[0003] Enlarging the electrodes in the lateral direction is often out ofthe question, on account of the associated increase in space taken up bysuch an electrode and the resultant increase in size of an electroniccomponent having a multiplicity of such electrodes.

[0004] For this reason, a technique for producing electrodes in whichtrenches are formed in a substrate has been developed, generally forelectrode arrangements in three-dimensional structures in which thestorage of the electrical charge carriers takes place in a stackedelectrode arrangement or by using the vertically running electrodesarranged in the trenches.

[0005] However, a three-dimensional structure of this type very quicklyencounters restrictions imposed by manufacturing technology, for exampleon account of the high aspect ratios of the trenches in which theelectrodes are formed.

[0006] For this reason, it has been attempted, both when usinghorizontally running capacitive surfaces and vertically runningcapacitive surfaces, i.e. of electrodes, to increase the effectiveelectrode surface areas while the macroscopic dimensions remain constantby roughening the respective electrode surfaces.

[0007] It is known to roughen an electrode surface, for example by meansof a special etching method in order to increase the porosity of thesurface or by applying additional spherical polysilicon structures tothe surface of the electrodes, which are also known as hemisphericalsilicon grains (HSG).

[0008] During the application of spherical polysilicon structures, it iscustomary for polysilicon, i.e. polycrystalline silicon, to be grownonto the electrode surface which is to be roughened from a solution.

[0009] Hemispherical islands with a diameter of usually approximately 30nm are formed on the electrode surface.

[0010] If, with a size of, for example, 30 nm, the density of thesepolysilicon islands is selected in such a manner that they positionthemselves at intervals of approximately 30 nm, it is possible toincrease the surface area by well over 20%.

[0011] However, a drawback of this procedure is that the grain size ofthe individual hemispherical polysilicon islands which are formed cannotbe controlled with accuracy and therefore an arrangement of this typecan only be achieved at all with considerable process engineeringproblems and therefore high costs.

[0012] In the text which follows, the hemispherical islands which areformed on an electrode surface are also known as clusters.

[0013] Furthermore, [1] has disclosed a cluster ion source which is usedto apply nickel clusters to a substrate surface.

[0014] In addition, it is known from [2] to form clusters from silveratoms on a graphite substrate.

[0015] Furthermore, [3] has disclosed a device for the mass separationof ion clusters, according to this particular example for the massseparation of silver clusters.

[0016] A further cluster ion source is described in [4].

[0017] [5] describes a process in which clusters of argon or phosphorusare applied to a polysilicon electrode.

[0018] Therefore, the invention is based on the problem of providing aprocess for producing a first electrode and a second electrode, as wellas an electronic component which is formed using this method and anelectronic memory element, in which it is possible for the grain size ofthe islands formed on the surfaces to be set more accurately.

[0019] The problem is solved by the process for producing a firstelectrode and a second electrode, the electronic component and theelectronic memory element having the features described in theindependent patent claims.

[0020] In a process for producing a first electrode and a secondelectrode, the first electrode and the second electrode are providedfrom an electrode material, which for example are integrated in asubstrate, preferably in a silicon substrate.

[0021] A cluster ion source is used to apply clusters of the electrodematerial to the first electrode and/or second electrode.

[0022] The electrode material may be either polycrystalline silicon,i.e. polysilicon, or a metal which can in principle be selected asdesired, such as nickel or silver.

[0023] The invention makes it possible, for the first time, to generatea beam profile in an accurately predeterminable manner, so that apredeterminable, if desired optimized, distribution of the clusterswhich are to be formed is ensured at the location of deposition, i.e. onthe electrode surface of an electrode which is to be roughened.

[0024] Furthermore, the size of the clusters which are to be formed canbe set very accurately.

[0025] A further advantage of the invention is that very accuratestructuring of the clusters on an electrode surface is made possible ina simple and therefore inexpensive way.

[0026] According to one configuration of the invention, the electrodematerial may also be doped silicon, i.e. silicon clusters are formed ona silicon electrode which is doped with correspondingly desired dopingatoms, the doping atoms being added to the ion beam, which is formed bythe cluster ion source, comprising generated silicon ions, in acondensation area of the cluster ion source, with the result that theelectrode material which is formed as clusters on the electrode surfacehas doped silicon clusters.

[0027] In principle, any desired electronic component which haselectrodes formed in this way can be formed from the electrodes.

[0028] A preferred application area for an electrode formed in this wayis electronic memories, for example an electronic memory element as adynamic random access memory, i.e. a RAM, or a flash EEPROM.

[0029] The further development of the doping of ion beams generated inthe condensation area of the cluster ion source allows very precise,simple and therefore inexpensive doping of the ion beam which wasoriginally generated, so as to form a cluster which contains both theions which were originally generated and the doping atoms, and thereforea cluster comprising a predeterminable number of doping atoms.

[0030] A further advantage of the invention resides in the fact that itis possible to form virtually (hemi-)spherical ion clusters, so that inthis way it is potentially possible to achieve a further increase in thesurface area of the electrode surface.

[0031] Furthermore, the invention makes it possible to produce clustergrain sizes which are significantly smaller than the grain sizes of theclusters which can be produced using the known method, so that eventhose areas of the electrode surface which adjoin relatively tightspaces may be suitable for the area of the electrode surface to beincreased

[0032] Exemplary embodiments of the invention are illustrated in thefigures and are explained in more detail below.

[0033] In the drawing:

[0034]FIG. 1 shows a cluster ion source for producing an electrodearrangement with a roughened electrode surface in accordance with afirst exemplary embodiment of the invention;

[0035]FIG. 2 shows a cross section through an electronic componenthaving two electrodes with a roughened electrode surface in accordancewith an exemplary embodiment of the invention; and

[0036]FIG. 3 shows a cluster ion source in accordance with a secondexemplary embodiment of the invention, for producing an electrodearrangement with a roughened electrode surface.

[0037]FIG. 1 shows a cluster ion source 100 in accordance with a firstexemplary embodiment of the invention.

[0038] The cluster ion source 100 has fundamentally the same structureas the cluster ion source described in [1].

[0039] In a housing 101 of the cluster ion source 100 there is a coolingcircuit 102 which is filled with cooling liquid, in accordance with thisexemplary embodiment with cooling water.

[0040] In the housing 101 there is a principal chamber 103, which, via afirst feed line 104, is fed with argon in the gas phase from a first gasvessel 105, which is filled with argon, as a continuous gas stream whichis regulated by means of a first mass flow regulator 106.

[0041] The argon gas is supplied under a pressure of T_(s1)=0.1 to 0.4kPa, which is generated in the principal chamber 103 by a turbomolecularpump 107 connected to the principal chamber 103.

[0042] Furthermore, the principal chamber 103 is fed, via a second feedline 108, with a doping gas, according to this exemplary embodiment withboron atoms, from a second gas vessel 109 which is filled with thedoping gas, as a continuous or discrete gas stream, which is regulatedby means of a second mass flow regulator 110.

[0043] The argon atoms are fired onto a sputtering target 111 made fromsilicon, and the doping gas is supplied through a condensation area 112of the cluster ion source 100, so that starting from the sputteringtarget 11 1 an ion beam 113 which contains both the silicon ions and theions of the doping gas is generated.

[0044] The ion beam is passed through a principal chamber opening 114with a diameter of approximately 5 mm, with the result that the ion beam113 is shaped in a predeterminable way.

[0045] Further adjustment of the beam profile of the ion beam 113 whichis formed takes place through further openings 115, 116, 117 in furtherelectrodes, the diameters of which openings are in each caseapproximately 5 mm, and by the different selection of the electricpotentials which are applied to the electrodes.

[0046] By means of the openings 115, 116, 117, in each case one chamberis defined between the respective openings 115, 116, 117, namely:

[0047] an area which is formed between the principal chamber 103 and thefirst opening 115, as a first auxiliary chamber 118,

[0048] a second auxiliary chamber 119, which is formed between the firstopening 115 and the second opening 116, and

[0049] a third auxiliary chamber 120 between the second opening 116 andthe third opening 117.

[0050] In the text which follows, the way in which the cluster ionsource 100 operates will be explained.

[0051] The free argon atoms are fired onto the sputtering target 111 viathe first feed line 104, and what are known as fast silicon ions aregenerated by firing onto the sputtering target 109 (silicon sputteringtarget).

[0052] These free silicon atoms drift through the condensation area 112in which they join together with the addition of the doping gascomprising boron atoms, to form larger groups, the ion clusters.

[0053] The ion clusters are typically formed in different sizes of fromonly a few atoms up to several thousand atoms, so that their diameterscan be kept smaller than the known diameters of silicon islands whichare generated in the usual way, for example using the known etchingprocess.

[0054] In this way, depending on the number of atoms, it is possible toform ion clusters of sizes of up to a few nanometers.

[0055] The diaphragms which define the openings 112, 113, 114, 115 areused to shape, accelerate or decelerate and focus the ion clusters toform a sharply defined ion beam 121.

[0056] The electric potential which is applied to the further electrodesand an electric field in front of the target, which is explained in moredetail below, makes it possible to deposit the ion clusters on a surfacewith an energy which can in principle be selected as desired.

[0057] The desired beam profile of the ion beam 113 can in principle beset as desired by suitably arranging the individual diaphragms andselecting the electric potential on these diaphragms.

[0058] The sharply defined ion beam 121, which still contains a largenumber of ion clusters of different sizes and therefore differentmasses, is passed through a mass separator 122, i.e. a device for massseparation, as described in [3].

[0059] The respective pressure prevailing in the auxiliary chambers 118,119, 120 is maintained by further turbomolecular pumps 123, 124, 125,which are in each case connected to the auxiliary chambers 118, 119, 120

[0060] The device for mass separation 122 has now made it possible togenerate a resulting ion beam 126 which has the ion clusters of thedesired mass and therefore of the desired size and the desired number ofions, i.e. the silicon atoms and the boron doping atoms.

[0061] The ion clusters are applied, on a substrate 128 which is held ina mount 127, to a first electrode 129 and a second electrode 130, bothof which have been prefabricated on the substrate 128.

[0062] Therefore, as illustrated in FIG. 2, the electrode surface 200 ofthe first electrode 129 and/or the electrode surface 201 of the secondelectrode 130 is roughened by the ion clusters 202 in a manner which canbe set very precisely, so that it is possible to generate an enlargedelectrode surface area 200 of the first electrode 129 and/or an enlargedelectrode surface area 201 of the second electrode 130.

[0063] In order not to destroy the electrode surface 200, 201, the ionclusters 202 which are to be deposited on the first electrode 129 and/orthe second electrode 130, before impinging on the respective electrodesurface 200, 201, are decelerated by means of an oppositely directedelectric field, diagrammatically indicated by an arrow 131 in FIG. 1

[0064] Furthermore, there is another turbomolecular pump 132, which iscoupled to the deposition chamber 133 in which the mount 127 isarranged.

[0065] The required magnitude of the electric field which is applied isdependent on the desired energy of the ion clusters just before theyimpinge on the electrode surface 200, 201 and is to be determinedexperimentally.

[0066] If one assumes a desired density of ion clusters 202 on anelectrode surface 200, 201 of approximately 10¹⁰ cm⁻², the result, withan assumed beam current of 1 pA, is a required radiation time ofapproximately 10 sec/mm

[0067] The structuring of the electrode surfaces 200, 201 is suitableboth for planar electrodes, as illustrated in FIG. 2, and for electrodeswhich are arranged in three-dimensional structures, for example intrenches of predeterminable depth.

[0068] For the deposition of ion clusters 202 on planar electrodesurfaces 200, 201, shaping of the ion beam with a beam profile which isas wide as possible at the location of deposition, i.e. of incidence onthe electrode surface 200, 201, is desirable.

[0069] During the deposition of ion clusters 202 within deep trenches, afocused, i.e. very sharply defined, cluster ion beam is desirable, whichcould either be directed in a controlled manner onto the location of thecorresponding trench at which the ion cluster 202 is to be deposited ineach case, or can be scanned across the entire electrode surface 200,201.

[0070] The corresponding variability of the beam profile of the ion beamcan be achieved by suitable selection of the ion-optical lenses whichare provided in the beam path of the ion beam, and of the voltagesapplied to the ion-optical lenses.

[0071] In principle, further electronic components of any desired form,preferably DRAM memory elements and flash EEPROM memory elements, can beproduced in further process steps from the electrodes 129, 130 which areembedded in the substrate 128 and have a roughened electrode surface200, 201.

[0072] According to an alternative embodiment of the invention, what isknown as a Wien filter is suitable as a device for mass separation 122.

[0073] A second cluster ion source 300 in accordance with a secondexemplary embodiment of the invention is illustrated in FIG. 3

[0074] Helium gas is passed by means of a feed line 303 into a principalchamber 302, which is provided with a cooling element 301, above acrucible 304, which contains silicon in the gas phase. The silicon gasin the crucible 104 is cooled by the helium and condenses to formclusters which are passed through a principal opening 305.

[0075] The clusters are passed into an area 306 which is held at apressure of approximately 10⁻⁴ mbar by means of a diffusion pump 307.

[0076] The cluster beam which is formed is doped, using doping atoms, inaccordance with this exemplary embodiment using boron atoms, by means ofa cathode plasma generator 308.

[0077] Alternatively, according to the invention it is also possible, inparticular, for the doping atoms

[0078] phosphorus atoms,

[0079] boron atoms,

[0080] arsenic atoms, but also

[0081] other doping atoms which are suitable for doping or further atomswhich are to be introduced into a cluster, to be used as doping atoms.

[0082] The doped cluster ion beam 309 is passed through a mouthpieceopening 310, for focusing the cluster ion beam 309, into a high-vacuumarea 311, which is connected to a further diffusion pump 312.

[0083] In the high-vacuum area 311, the doped cluster ion beam 309 isaccelerated and is focused further in an acceleration area 313 andoriented in an X-Y deflector 314.

[0084] A lens 315 focuses the doped cluster ion beam 309 further, andthe doped cluster ion beam which has been focused in this way is fed toa Wien filter 316 as mass selector.

[0085] A further Y-deflector 317 is used to direct the resulting clusterion beam into the centre of the mass selection opening 318 at the end ofa drift tube 319.

[0086] The mount 320 on which the substrate with the electrodes isarranged is positioned by means of a linear drive 321.

[0087] Furthermore, a valve 322 is provided between the drift tube 319and the deposition chamber 323, for the purpose of controlling thecluster ion beam emerging from the Wien filter 316 and the drift tube319.

[0088] The following publications are cited in this document:

[0089] [1] T. Hihara and K. Sumiyama, Formation and size control of a Nicluster by plasma gas condensation, Journal for Applied Physics, Volume84, No. 9, pp. 5270-5275, November 1998

[0090] [2] S. G. Hall, M. B. Nielsen and R. E Palmer, Energetic impactof small Ag clusters on graphite, Journal for applied physics, Volume83, No. 2, pp. 733-737, January 1998

[0091] [3] B. von Issendorff and R. E. Palmer, A new high transmissioninfinite range mass selector for cluster and nanoparticle beams, Reviewof Scientific Instruments, Volume 70, No. 12, pp. 4497-4501, December1999

[0092] [4] I. M. Goldby et al., Gas condensation source for productionand deposition of size-selected metal clusters, Rev. Sci. Instrum. 68,(9), 3327-3334, September 1997

[0093] [5] JP 9-232543 A

1. Process for producing a first electrode and a second electrode, inwhich the first electrode and the second electrode are provided from anelectrode material, in which a cluster ion source is used to applyclusters of the electrode material to the first electrode and/or thesecond electrode.
 2. Process according to claim 1, in which theelectrode material used is semiconductor material.
 3. Process accordingto claim 2, in which the electrode material used is silicon.
 4. Processaccording to one of claims 1 to 3, in which the electrode material usedis silicon doped with doping atoms.
 5. Process according to claim 4, inwhich the doping atoms are added to the ion beam which is formed by thecluster ion Source in the condensation area of the cluster ion source,with the result that the electrode material is provided.
 6. Electroniccomponent having a first electrode and a second electrode, in which thefirst electrode and/or the second electrode has/have been formed usingthe method according to one of claims 1 to
 5. 7. Electronic memoryelement having a first electrode and a second electrode, in which thefirst electrode and/or the second electrode has/have been formed usingthe method according to one of claims 1 to
 5. 8. Electronic memoryelement according to claim 7, in which the electronic memory element isa dynamic random access memory (RAM).
 9. Electronic memory elementaccording to claim 7, in which the electronic memory element is a flashEEPROM.